Mast cells and immunoglobulin E (IgE) play a pivotal role in allergies and asthma, and they are implicated in inflammation. In most species examined, mast cells localized in different tissues are functionally heterogeneous; their response varies markedly to secretagogues and inhibitors of secretion such as the anti-asthmatic drug cromolyn; some proliferate and secrete lymphokines in response to cross-linkage of IgE receptors or parasite infections and others do not. Associated with this functional heterogeneity is marked structural and biochemical specialization. This heterogeneity and the differentiation process that gives rise to it are best-defined in rodents, especially rats, where two mast cell subtypes are recognized, mucosal mast cells (MMC) and serosal mast cells. It is probable that the membrane physiology of mast cell subsets differs, and it is possible that such differences contribute to functional diversity. Antigen-driven calcium signaling, e.g., depends critically on membrane potential, and this in turn is regulated by ion channels. The long term goal of this work is to understand mast cell differentiation at the molecular level, and the focus here is on ion channel expression during mast cell differentiation. Although the ion channel repertoires of rat serosal mast cells and a transformed rat MMC are partially characterized, there are no corresponding data for normal rat MMC. Here we propose to use patch-clamp recording to characterize the ion channels present in primary rat MMC and bone marrow-derived mast cells (BMMC), a preparation of mast cells differentiated in vitro that closely resembles MMC. We will use digital imaging microscopy of fluorescent dyes to characterize IgE- mediated calcium signaling and membrane potential changes in MMC and BMMC. The possible role of mast cell K+ channels in regulation of membrane potential, calcium signaling, and secretion will be explored. Finally, selectively expressed ion channels will be tested as potential targets for pharmacological blockade of allergic and inflammatory mediator secretion, and for inhibition of interleukin-3 dependent proliferation of rat BMMC. This work will advance our understanding of mast cell heterogeneity and provide a missing chapter on the ion channel physiology of rat mucosal mast cells. It also could yield a new strategy for pharmacologic intervention in allergies or asthma.